Patent Literatures 1, 2 and 3 identified below disclose torque-distribution type power transmission devices in four-wheel drive vehicles. More specifically, Patent Literature 1 discloses that a clutch for connecting and disconnecting drive power transmission is provided between a center drive shaft for transmitting rotary motion of a drive source (engine) to rear wheels and a differential unit for the rear wheels, and that switching is made between two-wheel drive and four-wheel drive through connection and disconnection, by the clutch, of the drive power transmission. Further, Patent Literatures 2 and 3 disclose that clutches are provided in corresponding relation to left and right wheels in a differential unit for the rear wheels, and that drive power is distributed to the left and right wheels through connection and disconnection, by the individual clutches, of the drive power transmission. If abnormality, such as galling or a spike (i.e., sharp rise) in clutch-pressing hydraulic pressure, has occurred in any of the clutches taking part in drive power distribution between the front and rear wheels in such a four-wheel drive vehicle, the front wheels and the rear wheels would be undesirably interconnected directly (namely, undesired drive force would be transmitted to both or either of the left and right wheels by the clutch being locked in an ON or engaged state), so that torque may undesirably increase sharply. In such a situation, cases and gears may get broken, and oil leakage, fire, etc. may occur. As a fail-safe function to obviate such risks, it has been conventional to provide a torque fuse (i.e., most torsionally-frangible portion or fracturing portion for emergency) in a suitable portion of a power transmission shaft. Namely, the most torsionally-frangible portion or fracturing portion for emergency in the form of a notch (recess) or the like is formed in the suitable portion of the power transmission shaft, so that, when torque exceeding a predetermined critical value has been applied to the power transmission shaft, the power transmission shaft is broken or fractured at the notch (recess) so as to free the rotation of the shaft. Such a portion (i.e., most torsionally-frangible portion or fracturing portion for emergency), formed in the power transmission shaft in such a manner that it has a reduced strength as a design approach and thus can be fractured when torque exceeding an acceptable range has been applied, is called a torque fuse.
With the conventionally-known technique, however, the torque fuse is provided in a desired portion without appropriate consideration being made about the position where the torque fuse should be provided. Particularly, with the conventionally-known technique, such a torque fuse is not provided in a shaft portion that is to be connected with a rotation member (e.g., input-side rotation member) of the clutch; that is, it has been common to form the torque fuse in a shaft portion avoiding the connecting portion with the clutch. But, if the conventional torque-fuse-formed portion is disconnected or severed, the axis of the rotation member of the clutch would get misaligned, so that the clutch may irregularly whirling (move around violently) to prevent stable self-propelled rotation.